Uncharged nuclear particles (neutrons and gammas) are commonly detected in a useful form (i.e., an electric impulse) by first converting them to charged particles via an atomic or nuclear interaction, and then detecting the ionization trail generated by the (secondary) charged particle. Devices designed for uncharged particle detection have been relatively large due to the necessary volume of material needed for particle conversion, the array of photomultiplier tubes, and/or cryogenic cooling apparatus. These complications have precluded the development of a small and compact detector. This invention enables a very efficient detector to be manufactured at a reasonable cost, which can be made so small in size as to fit in a pocket, and which can operate at ambient temperatures for an indefinite length of time with minimal maintenance. Such devices as photomultiplier tubes and any kind of cryo system are unnecessary.
The basic concept of this invention is that, after efficiently converting the thermal neutron to an .alpha. particle using thin B.sup.10, a charged coupled device (CCD) can be used to detect the charged particles. Further, in order to reject false alarms and to permit detection to very low neutron flux levels, a stack of CCD arrays can be arranged so that their output can be processed to disregard higher energy particles of no interest to the intended measurements. Classically a CCD is used to detect visible or IR photons. However, it is also a good charged particle detector, but with poor energy resolution.
Although semiconductor materials (Ge, Si) which are used in CCDs have themselves been employed as radiation detectors, the CCD has not generally been used for charged particle detection because it provides no or poor energy resolution for KeV-MeV energy charged particles. This is the major reason why CCDs have been developed as highly sensitive visible light detection and imaging devices, but not for detection of charged particles. Interaction of charged particles with CCDs is regarded as a source of noise, not something to be detected or measured.
It is an object of this invention to make use of the extreme and well-understood sensitivity of the CCD to ionizing radiation for the detection of thermal neutron reaction products even at low neutron flux levels. Importantly, where a CCD is used for this purpose, neither energy resolution nor imaging is required. Thus simplified, CCD technology enables the construction of a compact, fully solid state, room temperature, neutron detector which is capable of registering nearly every incident neutron.
This device is also amenable to discrimination against energetic background radiation. While a single CCD pixel will stop alpha particles from a neutron of interest, cosmic rays, energetic gamma particles, and charged particles from other sources will penetrate several layers of CCD arrays. This will cause activation in several layers of CCD arrays. Coincidence measurements between adjacent arrays can be used to discriminate these types of particles or rays from those of interest to this detector, thereby to reject false alarms, and to permit detection to very low neutron flux levels.
Also, less energetic charged background particles, as, for example, from radon decay, can be eliminated by encasing the detector in a material which will stop all but the most energetic charged particles from entering the detector, but will have no effect on the neutrons which are to be detected. A very high degree of discrimination is thereby attainable.